This invention concerns injection molding and more particularly the valving used to control the flow of melted plastic into a mold cavity.
Injection molding of plastic involves forcing hot melted plastic into a mold cavity through a gate in an injection nozzle, and thereafter closing the gate in the injector nozzle to stop the flow of melt and allow separation of the mold plates and removal of the molded part. The gate is typically opened and closed by movement of a valve pin on and off a valve seat defining the gate. The pin is operated with an actuator such as shown in U.S. Pat. Nos. 5,670,190 and 5,067,893 to the present inventor.
In a variation of the arrangement of moving a valve pin on and off a valve seat, the pin can be formed with a reduced diameter section intermediate a full diameter end which enters into an opening in the mold plate. The hot melt is injected through the annular space formed by the reduced diameter section when the gate is open and into the mold cavity. The pin is retracted to bring the full diameter end into the gate and close off further flow of the melt.
This arrangement is sometimes referred to as "core ring" gating, as an annular space is defined around the pin when the gate is opened. This design is used for molded parts having a center hole, formed by the end of the valve pin, as the center hole is formed when the part is molded.
This arrangement is shown in Japanese published application no. 55061438 and also in U.S. Pat. No. 4,521,179.
This arrangement creates problems as the pin is mounted within the heated nozzle, and thus the pin end tends to be hotter than the mold plate in which cavity is formed. This in turn tends to cause the injected plastic adjacent the pin to remain molten for too long.
Heretofore, exotic pin and nozzle constructions have been employed in an attempt to prevent the hot pin end from interfering with the proper formation of the part.
It is the object of the present invention to provide a core ring pin gating arrangement which does not create the overheating problem described.